Distribution Of Groundwater Recharge Research Articles

GIS-Based Water Budget Framework for High-Resolution Groundwater Recharge Estimation of Large-Scale Humid Regions

Groundwater recharge plays a vital role in replenishing aquifers, which impacts on groundwater resources availability. This paper develops a geographic information system (GIS)-based water budget framework in conjunction with a hydrologic model to derive high-resolution spatial-temporal distribution of groundwater recharge for the Southern Hills aquifer system, southwestern Mississippi, and southeastern Louisiana, United States. The framework includes parallel programming to distribute the large amount of software runs to a cluster of supercomputers to expedite computation time. Under the parallel computation, the hydrologic models are calibrated by the USGS WaterWatch runoff dataset and verified by the moderate resolution imaging spectroradiometer (MODIS) evapotranspiration dataset. A map of recharge time lag is derived to understand travel time of infiltrated precipitation reaching the last soil layer. A recharge index map is derived to quantify the percentage of precipitation that becomes groundwater recharge. The results show that the mean annual recharge rate was estimated 47.5 mm/year which was 3.1% of the mean annual precipitation on the area of the Southern Hills aquifer system. High recharge rate was estimated at the outcrops of Miocene deposits in southwestern Mississippi, which are the recharge zones of the deep sands in southeastern Louisiana. This paper successfully demonstrated the applicability of the framework to groundwater recharge estimation on large-scale humid areas.

Evaluation of groundwater recharge estimates in a partially metamorphosed sedimentary basin in a tropical environment: application of natural tracers.

This study tests the representativeness of groundwater recharge estimates through the chloride mass balance (CMB) method in a tropical environment. The representativeness of recharge estimates using this methodology is tested using evaporation estimates from isotope data, the general spatial distribution of the potential field, and the topographical variations in the area. This study suggests that annual groundwater recharge rates in the area ranges between 0.9% and 21% of annual precipitation. These estimates are consistent with evaporation rates computed from stable isotope data of groundwater and surface water in the Voltaian Basin. Moreover, estimates of groundwater recharge through numerical model calibration in other parts of the terrain appear to be consistent with the current data in this study. A spatial distribution of groundwater recharge in the area based on the estimated data takes a pattern akin to the spatial pattern of distribution of the hydraulic head, the local topography, and geology of the terrain. This suggests that the estimates at least qualitatively predicts the local recharge and discharge locations in the terrain.

A Method of Estimating Conservative Potential Amount of Groundwater

지금까지 세계적으로 지하수 관리는 연간 지하수 함양량을 기반으로 한 안전채수량 정책에 의해 수행되어 왔다. 그러나 지하수 함양량은 시공간적으로 변동하므로 지속가능한 지하수자원을 평가하기 위해서는 동적 해석이 필요하다. 본 연구에서는 지표수-지하수 통합해석 모형인 SWAT-MODFLOW를 이용하여 경주지역의 지하수 함양량 공간분포를 산정하였다. 우리나라에서는 10년 빈도 갈수시 강수량에 함양계수를 곱하여 지역별 개발가능량을 산정하기에 본 연구에서는 기존 개발가능량을 평가하기 위해 빈도해석기법을 이용하였다. 소유역별 10년 빈도 갈수시의 지하수 함양량을 추정하는 보수적 방법을 제안하고 이를 기존 개발가능량과 비교하였다. 이같은 계산 절차를 통해 지하수 개발가능량을 산정하는 기존 절차의 한계를 합리적으로 제시할 수 있었다. By far, groundwater management has been conducted by 'safe yield' policy based on the estimation of annual average of groundwater recharge throughout the world. However, as groundwater recharge show spatiotemporal variation, dynamic analysis must be carried out to evaluate the sustainable groundwater resources. In this study, an integrated surface-groundwater model, SWAT-MODFLOW was used to compute the spatial distribution of groundwater recharge in Gyungju region. Frequency analysis is adopted to evaluate the existing values of potential amount of groundwater development which is made by the 10 year drought frequency rainfall multiplied by recharge coefficient. The conservative methods for estimating recharge rates of 10 year drought frequency in subbains are newly suggested and compared with the existing values of potential amount of groundwater development. This process will promote the limitations for existing precesses used for computing potential amount of groundwater development.

English

A quantitative evaluation of spatial and temporal distribution of groundwater recharge is a pre-requisite for the management of ground water resources system in an optimal manner. The amount of groundwater recharge depends upon the rate and duration of rainfall, as rainfall is the principal means for replenishment of moisture in the soil water system and recharge to ground water. This paper investigated the relationship between rainfalls and groundwater recharge within Ona River basin, southwest Nigeria, using soil moisture balance and water table fluctuation. Analysis of rainfall trends within the Ona River basin suggests that there is considerable high annual rainfall occurrence, with a mean of 1623.48. It must be noted that the mean annual lost due to evapotranspiration of 1361.68 mm is very high when compared to the rainfall (83.9%). The results obtained from the soil moisture balance when considering the three dominant soil types within the basin, that is, sandy loam, clay and find sand, having water capacity of root zone value of 70, 70, and 50 respectively, suggests that groundwater recharge follows a positive trend as the corresponding rainfalls. However, empirical relationships of: y = 0.540x – 606.2, with a coefficient of determination (r2) value of 0.719, for sandy loam and clay; and y = 0.552x – 621, with a coefficient of determination (r2) value of 0.726 for fine sand was established for the basin area. On the other hand, recharge ranging from 220.25 to 40.50 mm was computed from the water table fluctuation method. Key words: Rainfall–recharge relationship, soil types, soil moisture balance, Ona River basin, water table fluctuation.

基于GPR技术的地表水地下水的耦合计算及应用

地表水和地下水的转换及其过程耦合模拟，是水资源开发利用和科学评价的基础，为了准确模拟区域地下水动态变化，需要将地表水与地下水进行耦合计算。根据SWAT分布式模型的特点，以其水文响应单元(HRU)和MODFLOW模型的有限差分网格(CELL)作为基本交换单元，将SWAT模型计算的地下水补给量和潜水蒸发量引入MODFLOW模型的地下水补给(RCH)模块和潜水蒸发(EVT)模块中，并将其应用于西安市黑河流域的地下水模拟计算。基于GPR技术，对模拟结果进行检验。结果表明，SWAT模型与MODFLOW模型的耦合计算，能准确模拟和预测该地区的地下水资源及其地表水和地下水之间的相互作用。In recent years, water resources are increasingly affecting the global environment and economy. Conversion process and its coupling simulation of surface water and groundwater are the founda-tion of water resources development and utilization, and scientific evaluation. In order to simulate change in regional groundwater, the quasi-distributed watershed model, SWAT was coupled with the fully-distributed groundwater model MODFLOW in simulation. The hydrologic response units (HRU) in the SWAT model were exchanged with cells in the MODFLOW model. By using the HRU- CELL conversion interface, the distributed groundwater recharge rate and the groundwater eva-potranspiration calculated by SWAT were introduced into the RCH and EVT modules of MODFLOW. The application of groundwater simulation in the Xi’an Heihe River basin of Shaanxi Province demonstrated that the coupled SWAT-MODFLOW was capable of predicting the groundwater situ-ation, and of being used for investigating the interaction between surface water and groundwater.

Groundwater Recharge Rate and Zone Structure Estimation Using PSOLVER Algorithm

The quantification of groundwater recharge is an important but challenging task in groundwater flow modeling because recharge varies spatially and temporally. The goal of this study is to present an innovative methodology to estimate groundwater recharge rates and zone structures for regional groundwater flow models. Here, the unknown recharge field is partitioned into a number of zones using Voronoi Tessellation (VT). The identified zone structure with the recharge rates is associated through a simulation-optimization model that couples MODFLOW-2000 and the hybrid PSOLVER optimization algorithm. Applicability of this procedure is tested on a previously developed groundwater flow model of the Tahtalı Watershed. Successive zone structure solutions are obtained in an additive manner and penalty functions are used in the procedure to obtain realistic and plausible solutions. One of these functions constrains the optimization by forcing the sum of recharge rates for the grid cells that coincide with the Tahtalı Watershed area to be equal to the areal recharge rate determined in the previous modeling by a separate precipitation-runoff model. As a result, a six-zone structure is selected as the best zone structure that represents the areal recharge distribution. Comparison to results of a previous model for the same study area reveals that the proposed procedure significantly improves model performance with respect to calibration statistics. The proposed identification procedure can be thought of as an effective way to determine the recharge zone structure for groundwater flow models, in particular for situations where tangible information about groundwater recharge distribution does not exist.

통합수문모형을 이용한 제주 한천유역의 지하수 변동 특성 모의

To investigate groundwater variation characteristics in the Hancheon watershed, Jeju Island, an integrated hydrologic component analysis was carried out. For this purpose, SWAT-MODFLOW which is an integrated surface-groundwater model was applied to the watershed for continuous watershed hydrologic analysis as well as groundwater modeling. First, ephemeral stream characteristics of Hancheon watershed can be clearly simulated which is unlikely to be shown by a general watershed hydrologic model. Second, the temporally varied groundwater recharge can be properly obtained from SWAT and then spatially distributed groundwater recharge can be made by MODFLOW. Finally, the groundwater level variation was simulated with distributed groundwater pumping data. Since accurate recharge as well as abstraction can be reflected into the groundwater modeling, more realistic hydrologic component analysis and groundwater modeling could be possible.

Impacts of human activities on recharge in a multilayered semiarid aquifer (Campo de Cartagena, SE Spain)

AbstractThe development of intense agriculture in semiarid areas modifies intensity and spatial distribution of groundwater recharge by summing irrigation return flow to limited rainfall infiltration. Environmental tracers provide key information, but their interpretation is complicated by more complex groundwater flow patterns. In multilayered aquifers, the real origin of the groundwater samples is hard to assess because of local mixing processes occurring inside long‐screened boreholes. We use environmental tracers (14C, 13C, 2H, 18O, 3H) to investigate the long‐term evolution of recharge in the five‐layer Campo de Cartagena aquifer in South‐Eastern Spain, in addition to high‐resolution temperature loggings to identify the depth of origin of groundwater. Despite the complex background, this methodology allowed a reliable interpretation of the geochemistry and provided a better understanding of the groundwater flow patterns. The tritium method did not give good quantitative results because of the high variability of the recharge signal but remained an excellent indicator of recent recharge. Nonetheless, both pre‐anthropization and post‐anthropization recharge regime could be identified and quantified by radiocarbon. Before the development of agriculture, recharge varied from 17 mm.year‐1 at the mountain ranges to 6 mm.year‐1 in the plain, whereas the mean annual rainfall is about 300 mm. In response to the increase of agricultural activity, recharge fluxes to the plain were amplified and nowadays reach up to 210 mm.year‐1 in irrigated areas. These values are strengthened by global water budget and local unsaturated zone studies. Copyright © 2013 John Wiley & Sons, Ltd.

Application of the water balance model J2000 to estimate groundwater recharge in a semi-arid environment: a case study in the Zarqa River catchment, NW-Jordan

Pollution and overexploitation of scarce groundwater resources is a serious problem in the Zarqa River catchment, Jordan. To estimate this resource’s potential, the amount and spatial distribution of groundwater recharge was calculated by applying the hydrological model J2000. The simulation period is composed of daily values gathered over a 30-year period (July 1977 to June 2007). The figure finally obtained for estimated groundwater recharge of the Zarqa River catchment is 105 × 106 m3 per year (21 mm a−1). This is 19 % higher than the value previously assumed to be correct by most Jordanian authorities. The average ratio of precipitation to groundwater recharge is 9.5 %. To directly validate modelled groundwater recharge, two independent methods were applied in spring catchments: (1) alteration of stable isotope signatures (δ18O, δ2H) between precipitation and groundwater and (2) the chloride mass balance method. Recharge rates determined by isotopic investigations are 25 % higher, and recharge rates determined by chloride mass balance are 9 % higher than the modelled results for the corresponding headwater catchments. This suggests a reasonably modelled safe yield estimation of groundwater resources.

Modeling Groundwater-Soil-Plant-Atmosphere Exchanges in Fractured Porous Media

Models for the soil-plant-atmosphere system, especially large scale models, frequently ignore interactions with underlying groundwater, and ignore fractures or macropores which may strongly influence the system response. Unsaturated fractured porous media can exhibit a range of behavior, depending on both the characteristics of the porous matrix and the fractures, and the climatic conditions to which they are subject. A dominant characteristic in surficial fractured porous geological formations is the vertical distribution of fractures in the near surface region. This paper explores how these different modes of behavior operate at various field sites and under different climatic conditions. Two diverse fractured porous settings are considered: fractured glacial till of the semi-arid, seasonally frozen Canadian prairies and the Chalk in humid, temperate south east England. Interpretations from hypothetical hillslope scale model simulations provide insights into how the properties of the material, and in particular the distribution of the fractures and the matrix hydraulic conductivity, affect the spatial distribution of evapotranspiration and the timing, magnitude and spatial distribution of groundwater recharge. Such behavior is not captured in conventional large scale models which consider only a shallow, one-dimensional soil moisture balance.

Analysis of Land Use Changes on Groundwater Dynamics

Distributed physically-based models have the predictive capacity to assess the effect of land use changes on groundwater dynamics across a range of scales. MIKESHE, which represents state-of-art of distributed hydrological model, is applied to the Skjern catchment. It is utilized to evaluate hydrologic impacts of land use changes in a watershed. Our overall objective was to quantitatively evaluate the effects of land use changes on watershed hydrology within the 1175 km2 Skjern catchment in Denmark. The results show that the soil is unsaturated and has bigger storage capacity. The groundwater recharge distribution has seasonal characters like the runoff, which mainly concentrated in winter and spring and decreased in summer and autumn.

Hydraulic subsurface measurements and hydrodynamic modelling as indicators for groundwater flow systems in the Rotondo granite, Central Alps (Switzerland)

Regional groundwater flow in high mountainous terrain is governed by a multitude of factors such as geology, topography, recharge conditions, structural elements such as fracturation and regional fault zones as well as man-made underground structures. By means of a numerical groundwater flow model, we consider the impact of deep underground tunnels and of an idealized major fault zone on the groundwater flow systems within the fractured Rotondo granite. The position of the free groundwater table as response to the above subsurface structures and, in particular, with regard to the influence of spatial distributed groundwater recharge rates is addressed. The model results show significant unsaturated zones below the mountain ridges in the study area with a thickness of up to several hundred metres. The subsurface galleries are shown to have a strong effect on the head distribution in the model domain, causing locally a reversal of natural head gradients. With respect to the position of the catchment areas to the tunnel and the corresponding type of recharge source for the tunnel inflows (i.e. glaciers or recent precipitation), as well as water table elevation, the influence of spatial distributed recharge rates is compared to uniform recharge rates. Water table elevations below the well exposed high-relief mountain ridges are observed to be more sensitive to changes in groundwater recharge rates and permeability than below ridges with less topographic relief. In the conceptual framework of the numerical simulations, the model fault zone has less influence on the groundwater table position, but more importantly acts as fast flow path for recharge from glaciated areas towards the subsurface galleries. This is in agreement with a previous study, where the imprint of glacial recharge was observed in the environmental isotope composition of groundwater sampled in the subsurface galleries. Copyright © 2012 John Wiley & Sons, Ltd.

Coupled Simulation of Xinanjiang Model with MODFLOW

This paper investigates the probability of integrating the Xinanjiang model, the most widely used watershed hydrological model in China, with the fully distributed groundwater model MODFLOW for processing a coupled flow simulation. Based on the flow simulation of the Xinanjiang and the SWAT model in the case study area, the correlation between the groundwater recharge and the total flow runoff is analyzed, which is then applied to compute the groundwater recharge in subbasins in the Xinanjiang model. By applying a loosely coupled modeling approach, the spatially distributed groundwater recharge is introduced into MODFLOW with the RCH package via the interface of the subbasins in the Xinanjiang model and the cells in MODFLOW; thus the integration of the two models is successfully realized. The application of the integrated model simulation in Zhangji, a groundwater-dominated area in the Huai River basin, China, demonstrates that the coupled Xinanjiang-MODFLOW simulation produces reasonable groundwater levels comparing to the observations. However the study also shows the need for further research in regarding to the improvement on Xinanjiang model in order to enable its direct integration with MODFLOW. Furthermore, the approach presented in this paper is limited to a unidirectional coupling of Xinanjiang and MODFLOW, which only accounts for groundwater level simulations, while the possible feedbacks from the surface water is neglected. More efforts are required to further facilitate the applicability and capability of the models integration.

Groundwater Recharge, Evapotranspiration and Surface Runoff Estimation Using WetSpass Modeling Method in Illala Catchment, Northern Ethiopia

Hydrometeorological information is important in planning and management of natural resources. The northern Ethiopia in general and Illala sub-basin in particular is lacking reliable information with regard to groundwater recharge, evapotranspiration and surface runoff. The main objective of the study is to estimate the distributed groundwater recharge, surface runoff and evapotranspiration amount of Illala sub basin using WetSpass modeling method. Long term mean hydrometeorological data and physical characteristics of the catchment such as land use/land cover, soil type, topography, groundwater level and slope are used as an input to the model. The mean annual groundwater recharge, evapotranspiration and runoff were found to be 66, 440 and 40mm respectively. Accordingly, recharge accounts for 12% of the precipitation while the rest 81% and 7% becomes evapotranspiration and surface runoff respectively. The study area is characterized by low groundwater recharge due to the presence of high evapotranspiration rate associated with high temperature, dry wind, low rainfall and relative humidity though it is a little bit large compared to some parts of the northern Ethiopia. Keywords : Groundwater, WetSpass, Recharge, Illala, Catchment, Tigray, Ethiopia

Combining hydrological modeling and GIS approaches to determine the spatial distribution of groundwater recharge in an arid irrigation scheme

Accurate quantification of the rate of ground- water (GW) recharge, a pre-requisite for the sustainable management of GW resources, needs to capture complex processes, such as the upward flow of water under shallow GW conditions, which are often disregarded when esti- mating recharge at a larger scale. This paper provides (1) a method to determine GW recharge at the field level, (2) a consequent procedure for up-scaling these findings from field to irrigation scheme level and (3) an assessment of the impacts of improved irrigation efficiency on the rate of GW recharge. The study is based on field data from the 2007 growing season in a Water Users Association (WUA Sho- makhulum) in Khorezm district of Uzbekistan, Central Asia, an arid region that is characterized by a predominance of cotton, wheat and rice under irrigation. Previous quali- tative studies in the region reported irrigation water supplies far above the crop water requirements, which cause GW recharge. A field water balance model was adapted to the local irrigation scheme; recharge was considered to be a fraction of the irrigation water losses, determined as the difference between net and gross irrigation requirements. Capillary rise contribution from shallow GW levels was determined with the HYDRUS-1D model. Six hydrological response units (HRUs) were created based on GW levels and soil texture using GIS and remote sensing techniques. Recharge calculated at the field level was up-scaled first to these HRUs and then to the whole WUA. To quantify the impact of improved irrigation efficiency on recharge rates, four improved irrigation efficiency scenarios were devel- oped. The area under cotton had the second highest recharge (895 mm) in the peak irrigation period, after rice with 2,514 mm. But with a low area share of rice in the WUA of \1 %, rice impacted the total recharge only marginally. Due to the higher recharge rates of cotton, which is grown on about 40 % of the cropped area, HRUs with a higher share of cotton showed higher recharge (9.6 mm day -1 during August) than those with a lower share of cotton (4.4 mm day -1 ). The high recharge rates in the cotton fields were caused by its water requirements and the special treatment given to this crop by water management planners due to its strategic importance in the country. The scenario simulations showed that seasonal recharge under improved irrigation efficiency could potentially be reduced from 4 mm day -1 (business-as-usual scenario) to 1.4 mm day -1 (scenario with maximum achievable efficiency). The com- bination of field-level modeling/monitoring and GIS approaches improved recharge estimates because spatial variability was accounted for, which can assist water managers to assess the impact of improved irrigation effi- ciencies on groundwater recharge. This impact assessment enables managers to identify options for a recharge policy, which is an important component of integrated management of surface and groundwater resources.

A simple overland flow calculation method for distributed groundwater recharge models

A method to improve the calculation of overland flow in distributed groundwater recharge models is presented and applied to two sub-catchments in the Thames Basin, UK. Recharge calculation studies tend to simulate the runoff flow component of river flow in a simplistic way, often as a fraction of rainfall over a particular period. The method outlined in this study intends to improve the calculation of groundwater recharge estimates in distributed recharge models but does not present an alternative to complex overland flow simulators. This method uses seasonally varying coefficients to calculate runoff for specified hydrogeological classes or runoff zones, which are used to model baseflow index variations across the basin. It employs a transfer function model to represent catchment storage. Monte Carlo simulation was applied to refine the runoff values. Decoupling the runoff zones between the two sub-catchments produces a better match between the simulated and observed values; however, the difference between observed runoff and the simulated output indicates other factors, such as landuse and topographical characteristics that affect the generation of runoff flow, need to be taken into account when classifying runoff zones. British Geological Survey © NERC 2011. Hydrological Processes © 2011 John Wiley & Sons, Ltd.

Assessment of Groundwater Recharge Using GIS

Pressure on drinking water is increasing tremendously due to the increase in population in Kathmandu valley. Groundwater is serving as one of main source of water supply in the valley. Due to the scarcity of surface water and high demand for drinking water, excess extraction of groundwater than it replenishes may cause negative effect to Kathmandu valley like subduction. Thus, proper quantification of groundwater recharge must be done to de! ne sustainable extraction of the groundwater. This study is conducted with the aim to demonstrate the simple water balance model within the GIS environment in order to quantify the spatial distribution of groundwater recharge. The simple water balance model Thornthwaite and Marther (1955) was used to quantify the water balance components in the upper bagmati watershed. The study shows that the groundwater recharge is high at the northern part specifically where there is high water holding capacity.

Evaluating the use of a gridded climate surface for modelling groundwater recharge in a semi‐arid region (Okanagan Basin, Canada)

AbstractSpatially distributed groundwater recharge was simulated for a segment of a semi‐arid valley using three different treatments of meteorological input data and potential evapotranspiration (PET). For the same area, timeframe, land cover characteristics and soil properties, groundwater recharge was estimate using (i) single‐station climate data with monthly PET calculated by the Thornthwaite method; (ii) single‐station climate data with daily PET calculated by the Penman–Monteith method; and (iii) daily gridded climate data with spatially distributed PET calculated using the Penman–Monteith method. For each treatment, the magnitude and distribution of actual evapotranspiration (AET) for summer months compared well with those estimated for a 5‐year crop study, suggesting that the near‐surface hydrological processes were replicated and that subsequent groundwater recharge rates are realistic. However, for winter months, calculated AET was near zero when using the Thornthwaite PET method. Mean annual groundwater recharge varied from ∼3·2 to 10·0 mm when PET was calculated by the Thornthwaite method, and from ∼1·8 to 7·5 mm when PET was calculated by the Penman–Monteith method. Comparisons of bivariate plots of seasonal recharge rates estimated from single‐station versus gridded surface climate reveal that there is greater variability between the different methods for spring months, which is the season of greatest recharge. Furthermore, these seasonal differences are shown to provide different results when compared to the depth to water table, which could lead to different results of evaporative extinction depth. These findings illustrate potential consequences of using different approaches for representing spatial meteorological input data, which could provide conflicting predictions when modelling the influence of climate change on groundwater recharge. Copyright © 2010 John Wiley & Sons, Ltd.

Assessing distributed groundwater recharge rate using integrated surface water-groundwater modelling: application to Mihocheon watershed, South Korea

A method of estimating groundwater recharge, based on water-balance components using the SWAT-MODFLOW model (an integrated surface water-groundwater model), is described. A multi-reservoir storage routing module is suggested instead of a single storage routing module in SWAT; this represents a more realistic delay in the travel of water through the vadose zone. By using this module, the parameter related to the delay time can be optimized by checking the correlation between simulated recharge and observed groundwater levels. The final step of this procedure is to compare simulated groundwater levels as well as the simulated watershed stream flow with the observed groundwater levels and watershed stream flow. This method is applied to the Mihocheon watershed in South Korea to estimate spatio-temporal groundwater recharge distribution. The computed annual recharge rate is compared with the independently estimated recharge rate using BFLOW. The hydrologic modelling results show that the annual average recharge rate should be estimated by a long-term continuous simulation with a distributed hydrologic modelling technique.

The spatial and temporal variability of groundwater recharge in a forested basin in northern Wisconsin

AbstractRecharge varies spatially and temporally as it depends on a wide variety of factors (e.g. vegetation, precipitation, climate, topography, geology, and soil type), making it one of the most difficult, complex, and uncertain hydrologic parameters to quantify. Despite its inherent variability, groundwater modellers, planners, and policy makers often ignore recharge variability and assume a single average recharge value for an entire watershed. Relatively few attempts have been made to quantify or incorporate spatial and temporal recharge variability into water resource planning or groundwater modelling efforts. In this study, a simple, daily soil–water balance model was developed and used to estimate the spatial and temporal distribution of groundwater recharge of the Trout Lake basin of northern Wisconsin for 1996–2000 as a means to quantify recharge variability. For the 5 years of study, annual recharge varied spatially by as much as 18 cm across the basin; vegetation was the predominant control on this variability. Recharge also varied temporally with a threefold annual difference over the 5‐year period. Intra‐annually, recharge was limited to a few isolated events each year and exhibited a distinct seasonal pattern. The results suggest that ignoring recharge variability may not only be inappropriate, but also, depending on the application, may invalidate model results and predictions for regional and local water budget calculations, water resource management, nutrient cycling, and contaminant transport studies. Recharge is spatially and temporally variable, and should be modelled as such. Copyright © 2009 John Wiley & Sons, Ltd.